1. Field of the Invention
The present invention relates to a solid-state laser device which is pumped by light output from a laser diode (hereunder abbreviated to "LD" and "LD-pumped solid-state laser device", respectively), and more particularly to an end pumping system-based LD-pumped solid-state laser device capable of inducing high-power optical output in a TEM.sub.00 mode in which specific TEM waves which are electromagnetic waves propagate through wave guides or cavities).
2. Description of the Related Art
With conventional LD-pumped solid-state laser devices, in order to produce TEM.sub.00 -mode laser light with high efficiency, the end pumping system is often used which provides a higher efficiency of overlap between the light from an LD for pumping a laser crystal (hereunder abbreviated to "LD-light") and the laser beam oscillating in a resonator than the side pumping system. With end pumping system-based LD-pumped solid-state laser devices, it is necessary to efficiently reduce the diameter of the beams of the LD-pumping light which are output from the stripe section of the LD with a variety of large divergence angles so as to match the diameter of the TEM.sub.00 -mode laser light in the resonator.
As disclosed in Japanese Unexamined Patent Application Disclosure HEI 6-347609, LD-pumped light is usually focused through a condenser lens such as a gradient-index lens (hereunder abbreviated to "GRIN lens") or focused through a single convex lens as the condenser lens. In addition, in cases where the LD-pumped light must be focused to a small cross section, an optical lens system provided by combining a plurality of convex lenses and/or concave lenses is used to focus the LD-pumped light.
Here, this type of conventional LD-pumped solid-state laser devices is also disclosed in, for example, "High-power Nd:YAG laser end pumped by a cw, 10 mm.times.1 .mu.m aperture, 10-W laser-diode bar" in OPTICAL LETTERS, 16, No. 5, Mar. 1, 1991; and "7.6 W of continuous-wave radiation in a TEM.sub.00 mode from a laser-diode end-pumped Nd:YAG laser" in OPTICAL LETTERS, 17, No. 14, Jul. 15, 1992, in addition to the document mentioned above.
The conventional LD-pumped solid-state laser device disclosed in the former document uses a plurality of cylindrical lenses or rod lenses to focus LD-pumped light. In contrast, the conventional LD-pumped solid-state laser device disclosed in the latter document uses a bundle of fibers to focus LD-pumped light.
When such conventional LD-pumped solid-state laser devices are applied to single stripe-type LDs, the optical diameter of the LD-light may be reduced to approximately 100-200 .mu.m, though the focused spot thereof is elliptical. In order to increase the output from single stripe-type LDs, however, they must be configured so as to focus outputs from a plurality of LD light sources coupled with respective optical fibers through a lens, as disclosed in Japanese Unexamined Patent Application Disclosure HEI 4-320383, or a bundle of fibers connected to output sections of a plurality of LD light sources must be used, as disclosed in Japanese Unexamined Patent Application Disclosure HEI 1-251678, in either case of which the overall coupled optical system inevitably becomes complicated and larger.
Japanese Unexamined Patent Application Disclosure HEI 2-146782 also discloses another conventional LD-pumped solid-state laser device which uses a focusing optical system equipped with a fiber-coupled-type pumping optical system which focuses LD-pumping light output through a core of approximately 100 .mu.m in diameter at a ratio of 1:1 or less. In particular, the incidence section of the optical fiber may take in pumped light even with a relatively large beam spot by using an optical fiber equipped with a tapered member.
With this conventional LD-pumped solid-state laser device, however, since the LD-pumping light emitted through the emission end of the optical fiber diverges at a large angle, the focusing optical system placed downward from the emission end of the optical fiber must have a microscopic objective lens-level numerical aperture (NA) in order to minimize the coupling loss. Particularly, a complicated and large optical system is needed in order to reduce the diameter of the pumped light so as to roughly match the core diameter of the fiber at the emission end. This also results in a lower transmittance of the pumped light and a higher manufacturing cost.
In addition, although generally it is extremely difficult to apply anti-reflective coats to both ends of flexible optical fibers, the output of the LD-pumped light is inevitably lost when no anti-reflective coats are applied to both ends of the optical fiber. For example, when an optical fiber formed of quartz is used, the LD-pumped light is lost by approximately 8% at both ends thereof.
In addition, in the case of the conventional LD-pumped solid-state laser device, dust tends to deposit at the output end of the optical fiber, and thus the output end of the optical fiber tends to be damaged.